1. Field of the Invention
The present invention primarily pertains to the field of watercraft and boating. More particularly, many aspects of the present invention pertaining to controlling the pitch attitude of powered watercraft moving through the water.
2. Related Art
Watercraft “porpoising” refers to excessive rise-and-fall motion of a boat while motoring across a body of water at otherwise steady velocity. Virtually every recreational boater experiences it from time to time without much bother, but it can be dangerous, especially if it becomes harmonic with a series of waves. To make matters worse, porpoising can increase unexpectedly and uncontrollably even though engine speed and other boat controls are being held constant.
Consider a traditional V-shaped hull moving on a straight path through the water. Ideally the boat reaches the intended speed and everything stays smooth and steady. A large body of water, however, is rarely smooth, and the nautical forces acting on a boat are anything but steady. Buoyancy and drag forces vary depending largely on how much of the hull is submerged. Meanwhile, the thrust from the boat's propulsion system may be fixed relative to the boat, but if a wave changes the boat's pitch, then the thrust pushes the hull out of or into the water. So, once a sizeable waves changes the boat's pitch and position relative to the water, the forces on the hull of the boat are dramatically varied.
The boat's pitch begins to oscillate as the boat rises and falls, because fore and aft buoyancy forces against the hull start varying dramatically once the oscillating motion of porpoising has commenced. As the bow moves down into the water, the bow area displacement and the surface area impinged by moving water both increase, and generate upward forces on the bow. When the upward forces from the water overcome the downward force from the propulsive thrust, the bow pitches up. As it does, the bow displacement decreases and the lifting forces decrease, but the propulsive thrust lifting the stern increases. Thereafter, propulsive thrust and inertia drive the bow back into the water below a steady state displacement level. Hence, without intervention, the porpoising cycle continues.
Mild to moderate porpoising can adversely affect fuel consumption, steering, passenger comfort, engine wear, and other matters. In severe porpoising, the pitch oscillations may damage equipment and injure passengers and crew. A control system that can reduce or prevent porpoising is desirable to avoid its adverse effects.
Modulating the available control mechanisms such as trim tabs or thrust adjustments (i.e., throttle or cruise control interventions) is also difficult to achieve accurately because the resulting forces act on multiple axes that are displaced from the boat's center of gravity. If trim tabs or buoyancy propels the stern upward from the water, then the bow is forced down. There are multiple factors that affect porpoising including boat speed, hull design, thrust angle, propeller type, boat loading, wind, waves, and more
Even assuming that porpoising is already being managed to a minimum, watercraft operators such as those using recreational boats may also wish to control the boat to achieve a particular pitch attitude of their boats. While such needs arise in various circumstances, ski boat operators for instance may want to alter the size of the boat's wake. In wakeboarding, a large wake allows the wakeboarder to jump higher in the air. However, if the boat creates large wakes while transiting to a wakeboarding area, the wake may damage moored or docked vessels, damage piers and shoreline, or disturb other boaters. A small wake may be desirable when transiting, trolling, water skiing, wakeskating, or performing other activities. A smaller wake contains less energy, and thus carries less risk of damaging impinged structures. Further, for a given speed, a smaller wake indicates less propulsive energy is being diverted into wave-making energy, and thus reflects increased fuel efficiency. Some existing systems and methods enable watercraft operators to vary the craft's attitude, but each has significant limitations such as cost, weight, inflexibility, slow response, excess operator intervention, unpredictable results, and the like.